Bulk synthesis of semiconductor wires has been traditionally achieved using several variations of metal catalyzed techniques, such as vapor-liquid-solid (VLS) synthesis. In conventional metal-catalyzed nanowire synthesis techniques used for producing semiconductor (e.g., silicon) nanowires, each wire grows from a single particle of gold, cobalt, nickel, or other metal. A vapor-phase silicon-containing species transported to the catalyst inside a high-temperature furnace reacts on the surface of the catalyst, is transported, and precipitates to form silicon nanowires. In the VLS technique, the catalyst nanoparticles are in the liquid form; in analogy to the VLS process, nanowires can be grown using metal catalyst nanoparticles that remain in the solid state during nanowire growth.
Silicon nanowires produced by the conventional VLS and related processes are composed of a single crystal. In the conventional process, the size of the catalytic particle controls the diameter of the nanowire grown from it. Thus, in order to obtain a uniform nanowire diameter distribution, monodispersed catalyst particles need to be created on a solid substrate. However, creation of nanometer-size catalyst particles is a non-trivial task. The nanoparticles can be formed by deposition techniques, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). Although they can be registered to previously formed patterns, creating these patterns requires additional processing, usually involving costly lithography. In addition, conventional lithography processes cannot readily form nanoparticles of the desired small dimensions.
The above-referenced application Ser. No. 10/281,678 provides a solution to the foregoing problem. A variety of embodiments are disclosed and claimed. In one embodiment, the formation of an ordered array of catalyst nanoparticles is achieved by imprinting, generally using two steps of imprinting at an angle, e.g., orthogonal, to each other. The pattern of nanocrystals is then used to catalyze the growth of nanowires, starting at the position of the nanoparticles.
The foregoing application describes methods for forming nanowires comprising a single material, such as Si or Ge. However, other researchers have shown that the composition of the nanowires can be changed during growth so that different materials are at different positions along the length of the nanowire.
There is a need to provide a method for forming solid arrays of nanowires, comprising alternating regions of different compositions. Such arrays would find use in a variety of applications, including, but not limited to, quantum dots and photonic bandgap crystals.